The recovery of olefins such as ethylene and propylene from gas mixtures is an economically important but highly energy intensive process in the petrochemical industry. These gas mixtures are produced by hydrocarbon pyrolysis in the presence of steam, commonly termed thermal cracking, or can be obtained as offgas from fluid catalytic cracking and fluid coking processes. Cryogenic separation methods are commonly used for recovering these olefins and require large amounts of refrigeration at low temperatures.
Olefins are recovered by condensation and fractionation from feed gas mixtures which contain various concentrations of hydrogen, methane, ethane, ethylene, propane, propylene, and minor amounts of higher hydrocarbons, nitrogen, and other trace components. Methods for condensing and fractionating these olefin-containing feed gas mixtures are well-known in the art. Refrigeration for condensing and fractionation is commonly provided at successively lower temperature levels by ambient cooling water, closed cycle propylene and ethylene systems, and work expansion or Joule-Thomson expansion of pressurized light gases produced in the separation process. Recent improvements in cryogenic olefin recovery methods have reduced energy requirements and increased recovery levels of ethylene and/or propylene.
One improvement to the cryogenic separation section of a conventional ethylene recovery process is described in U.S. Pat. No. 4,002,042 whereby the final feed gas cooling and ethylene condensing step, between about -75.degree. F. and -175.degree. F., is performed in a dephlegmator-type heat exchanger. This provides a much higher degree of prefractionation as the ethylene-containing liquids are condensed out of the cold feed gas, since the dephlegmator can provide 5 to 15 or more stages of separation, as compared to the single stage of separation provided by a partial condenser. As a result, significantly less methane is condensed from the feed gas and sent to the demethanizer column and refrigeration energy requirements for both feed cooling and demethanizer column refluxing are reduced. The multi-stage dephlegmator also condenses the ethylene at warmer temperatures than the single-stage partial condenser, which provides additional savings in refrigeration energy.
Further improvements to the cryogenic separation and cold fractionation sections of the conventional process are described in U.S. Pat. Nos. 4,900,347 and 5,035,732. Feed gas cooling for ethylene recovery below about -30.degree. F. is done in a series of at least two dephlegmators, for example, a warm dephlegmator and a cold dephlegmator, and the demethanizer column is split into a first (warm) demethanizer column and a second (cold) demethanizer column. The warm dephlegmator condenses and prefractionates essentially all of the propylene and heavier hydrocarbons remaining in the -30.degree. F. feed gas along with most of the ethane and this liquid is sent to the warm demethanizer column. Reflux for the warm demethanizer column typically is provided by condensing a portion of the overhead vapor against propylene or propane refrigeration at -40.degree. F. or above. The cold dephlegmator condenses and prefractionates the remaining ethylene and ethane in the cold feed gas and this liquid is sent to the cold demethanizer column. Reflux for the cold demethanizer column is typically provided by condensing a portion of the overhead vapor using ethylene refrigeration at about -150.degree. F.
U.S. Pat. No. 5,082,481 discloses a variation of the conventional process whereby a portion of the hydrogen to be used as fuel, for example 20%, is removed from the cracked gas feed at near ambient temperature prior to cooling. This allows the condensation and separation of the hydrocarbons to be carried out at higher temperatures, with a corresponding reduction in refrigeration energy requirements. Hydrogen product is produced by means of a low temperature hydrogen recovery system.
A process is described in U.S. Pat. No. 4,732,583 in which a hydrogen-containing stream is separated in a membrane separator into a high purity hydrogen stream and a low purity hydrogen stream prior to processing the low purity hydrogen stream in a cryogenic separation unit to produce a second high purity hydrogen stream without depressurization. This process relates to the cryogenic purification of hydrogen at high pressures, near the critical pressure of the hydrogen-containing stream.
U.S. Pat. No. 5,053,067 discloses a similar process whereby a portion of the hydrogen in a refinery offgas is removed prior to fractionation such that the overhead condenser of the fractionation column can be operated at a temperature of -40.degree. F. or warmer to utilize high level refrigeration (e.g., propylene refrigeration). This process relates to the recovery of C.sub.3 or heavier hydrocarbon components from refinery offgas.
Nitric oxide (NO) is present in olefin-containing feed gas obtained from fluid catalytic cracking and fluid coking processes, and may be present in cracked gas obtained by thermal cracking. NO can enter the cryogenic section of an olefin recovery plant and cause the formation and buildup of unstable nitrogen compounds such as nitrosogums and ammonium nitrite. Such accumulated nitrogen compounds can react explosively at certain conditions and severely damage process equipment. These compounds can accumulate in the low pressure methane vaporization circuit(s) of the low temperature hydrogen recovery system heat exchangers and the demethanizer column feed liquid rewarming circuit(s) in the cold ethylene recovery partial condensers. These circuits contain liquid streams which are introduced at temperatures below -166.degree. F. (-110.degree. C.) which is believed to be the critical upper temperature limit for the formation of these unstable nitrogen compounds. This safety problem is discussed in an article by S. Shelly entitled "Reengineering Ethylene's Cold Train" in Chemical Engineering, January 1994, pages 37-41.
The development of new processing options, particularly in the initial gas cooling and condensation steps prior to final distillation, is desirable to improve the efficiency of olefin recovery systems. In particular, it is beneficial to reduce the amount of hydrogen in the feed to the lower temperature processing steps operating below -100.degree. F. and especially below -150.degree. F. This, in turn, reduces refrigeration at the lowest temperature levels required for high ethylene recovery. In addition, it is desirable to operate at conditions which minimize or eliminate the formation and accumulation of unstable nitrogen compounds in the olefin recovery system. The invention described in the following specification and defined in the appended claims addresses these needs and provides an improved method for the initial cooling and condensation of olefin-containing feed gas prior to low temperature fractionation.